APRS ANTENNAS by ED LAWRENCE WA5SWD SIMPLIFIED & TRANSMISSION LINES

Simple VHF or UHF Vertical Antenna Typically 1/4 wavelength tall. Has an input impedance of about 36 . Normally fed with 50  Coax with ground conductor connected to earth ground. Best SWR 1.5:1 Usually requires an array of radials to develop a better ground plane. (Marconi antenna) For APRS uses it should be vertically polarized

Monopole Antenna ¼ wavelength fed at one end Fed with unbalanced feedline with ground conductor connected to earth ground. In practice it usually requires an array of radials to develop a better ground plane. (Marconi antenna) When used at low frequencies the field should be vertically polarized and antenna could be a tower. The tower is ground insulated and fed at a point above ground with a Gamma match. Z increases upward.

APRS Ground Plane Antenna Can use a COUNTERPOISE system of radials cut to ¼ wavelength to develop ground plane elevated above earth. If used in a mobile application the roof of the vehicle can serve as a ground plane.

CLASSIC COAX FED 1/4 WAVELENGTH MONOPOLE NOTE THE RADIALS AT RIGHT ANGLES EXPECT THE INPUT IMPEDANCE TO BE ABOUT 36 OHMS

5/8 th wavelength Antenna Application as a mobile or base station antenna.. Omnidirectional response in horizontal plane. Advantage is realized in the concentration of low angle radiation in horizontal direction. Does not require as good a ground plane because feedpoint Z at 5/8 th wavelength is higher therefore lower current. Z is lowered to match 50 ohm feedline by matching section.

Collinear Array Two or more half wavelength sections. A broadside array because the axes of the elements are along same line. Half wave sections are linked by ¼ wave transmission lines. They develop a phase reversal to keep all dipoles in phase. Usually vertical with an omnidirectional pattern in the horizontal plane with a narrow angle of radiation in the vertical. What would be a good application for this system?

THIS EXAMPLE IS A DIPOLE, NOT A MONOPOLE

NOT your usual APRS antenna!

50  COAX USED AT MHz RG 58/U6.8 dB loss/100 ft. RG 58/U foam4.7 dB loss/100 ft. RG 8/U 3.2 dB loss/100 ft. RG 8/U foam2.2 dB loss/100 ft. RG 8X3.7 dB loss/100 ft. RG 213 USE RG 8 VALUES